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Abstract:

A device includes a substrate 5, an emission source 1 emitting an
acoustic beam from the first surface 7 of the substrate 5 towards the
second surface 6 of the substrate 5, first reflection element 11 arranged
so that, after reflection, the beam has a propagation direction
substantially parallel to the second surface 6 and through a study area
3, and second reflection element 12 for reflecting the beam after
crossing the study area 3 in the direction of the first surface, towards
a receiver 10. The device is useful in the field of bioMEMS and
labs-on-a-chip, and in particular makes it possible to leave one of the
surfaces of the substrate free, for example for observation of the study
area in parallel by element of an optical microscope. Moreover, the
device facilitates the production of elements for the focussing or
divergence (lenses) or reflection (mirror) of the acoustic beam.

Claims:

1. Device for studying a study area (3) by means of an acoustic wave,
comprising a substrate provided with distinct first (7) and second (6)
surfaces, characterized in that it comprises: an emission source (1)
arranged to emit an acoustic beam originating from the first surface (7)
in the direction of the second surface (6), first reflection means (11)
arranged to reflect said beam originating from the first surface, so
that, after reflection, the beam has a propagation direction
substantially parallel to the second surface (6) and crosses the study
area (3), second reflection means (12) arranged to reflect the acoustic
beam in the direction of the first surface (7) after the beam has crossed
the study area (3), and a receiver (10) arranged to receive said acoustic
beam reflected by the second reflection means (12).

2. Device according to claim 1, characterized in that the emission source
(1) is situated on the first surface (7).

3. Device according to claim 1, characterized in that the receiver (10)
is situated on the first surface (7).

4. Device according to claim 1, characterized in that the first and/or
second reflection means (11; 12) comprise a mirror formed by an interface
of materials.

5. Device according to claim 4, characterized in that the first and/or
second reflection means (11; 12) comprise a mirror formed by an interface
with the substrate.

6. Device according to claim 4, characterized in that the first and/or
second reflection means (11; 12) comprise a mirror formed by an
air/substrate interface.

7. Device according to claim 4, characterized in that it also comprises
at least one additional layer of material or a set of several additional
superimposed layers of different materials, this layer or these layers
being arranged on the substrate, so that the first and/or the second
reflection means comprise a mirror formed by an interface of materials
between the substrate and this additional layer or this set of several
additional layers.

8. Device according to claim 7, characterized in that each additional
layer is selected from: a layer of metal such as preferably copper, zinc,
tin, titanium, or a layer of dielectric material, preferably comprising
silicon, preferably a silicon oxide such as silicon monoxide SiO or
silica SiO.sub.2.

9. Device according to claim 7, characterized in that the thickness of
each additional layer is preferably comprised between 0.1 micrometre and
10 micrometres, more preferentially between 0.5 micrometres and 5
micrometres.

10. Device according to claim 1, characterized in that the first and/or
second reflection means (11; 12) comprise a recess formed on the second
surface (6) of the substrate.

11. Device according to claim 1, characterized in that the first and/or
second reflection means (11; 12) comprise a mirror inclined at 45.degree.
with respect to the second surface (6).

12. Device according to claim 1, characterized in that the emission
source (1) and/or the receiver (10) comprise piezoelectric transducers.

13. Device according to claim 1, characterized in that the substrate (5)
is a silicon-based substrate.

14. Device according to claim 1, characterized in that the study area (3)
comprises part of a microfluidic channel (13) situated on the second
surface (6).

15. Device according to claim 1, characterized in that it comprises at
least one lens (4), each lens (4) being arranged in order to be passed
through by the acoustic beam between the first reflection means (11) and
the study area (3) and/or between the study area (3) and the second
reflection means (12), preferably formed in the substrate (5).

16. Device according to claim 15, characterized in that the at least one
lens (4) has a form or a curvature that does not vary along an axis
substantially perpendicular to the second surface (6).

17. Device according to claim 15, characterized in that the at least one
lens (4) is formed in a wall of the microfluidic channel (13).

18. Device according to claim 1, characterized in that the emission
source (1) is distinct from the receiver (10), and in that the first
reflection means (11) are distinct from the second reflection means (12).

19. Device according to claim 1, characterized in that the first and
second reflection means (11;12) are merged.

20. Device according to claim 1, characterized in that the second
reflection means (12) and the receiver (10) are arranged in order to
receive a beam diffracted in the study area (3).

21. Device according to claim 1, characterized that it comprises several
emission sources (1) and several receivers (10), each receiver (10) being
associated with one of the emission sources (1) and being arranged in
order to receive an acoustic beam emitted by the emission source (1) with
which it is associated, then reflected by the first reflection means
(11), then reflected by the second reflection means (12) after having
crossed the study area (3).

22. Device according to claim 21, characterized in that the emission
sources (1) are aligned so that different acoustic beams emitted by the
different emission sources (1) cross the study area (3) at different
heights.

23. Device according to claim 21, characterized in that the projections
of the emission sources (1) and of the receivers (10) on a plane parallel
to the second surface (6) and crossing the study area (3) are distributed
around the study area (3).

24. Device according to claim 1, characterized in that the first and/or
second reflection means (11; 12) comprise a curved mirror.

25. Method for studying a study area (3) by means of an acoustic wave,
comprising: the emission (1) of an acoustic beam originating from a first
surface (7) of a substrate in the direction of a second surface (6) of
this substrate, the first and second surfaces being distinct, a first
reflection by first reflection means, said first reflection reflecting
said beam originating from the first surface, so that, after reflection,
the beam has a propagation direction substantially parallel to the second
surface (6) and crosses the study area (3), a second reflection by second
reflection means, said second reflection reflecting the acoustic beam in
the direction of the first surface (7) after the beam has crossed the
study area (3), and the reception, by a receiver, of the acoustic beam
reflected by the second reflection means (12).

26. Method according to claim 25, characterized in that it comprises the
passing of the acoustic beam through at least one lens (4), each lens (4)
being arranged in order to be passed through by the acoustic beam between
the first reflection means (11) and the study area (3) and/or between the
study area (3) and the second reflection means (12).

27. Method according to claim 25, characterized in that it comprises the
reception, by the second reflection means (12) and by the receiver (10),
of an acoustic beam diffracted in the study area (3).

28. Method according to claim 25, characterized that it comprises several
emissions of acoustic beams by several sources (1) and several receptions
of acoustic beams by several receivers (10), each receiver (10) being
associated with one of the emission sources (1) and receiving an acoustic
beam emitted by the emission source (1) with which it is associated, then
reflected by the first reflection means (11), then reflected by the
second reflection means (12) after having crossed the study area (3).

29. Method according to claim 28, characterized in that the different
acoustic beams emitted by the different emission sources (1) cross the
study area (3) at different heights.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a device and a method for studying
a study area by means of an acoustic wave.

[0002] The field of the invention is more particularly but not
limitatively that of bioMEMS (bio-MicroElectroMechanicalSystem) and
labs-on-a-chip or that of the integration of acoustic components on
wafers, electronic and microfluidic components, guided and localized
acousto-optic interactions and the study of phonic networks on wavelength
scales of the order of a micron.

[0003] The invention responds in particular to a growing demand from
biologists to have integrated sensors providing dynamic information on
mechanical properties of biological cells.

STATE OF THE PRIOR ART

[0004] Usually, the production of acoustic components on a wafer or thin
substrate requires the deposition of at least one acoustic beam emission
source on a first surface of a thin substrate, which leads to a
constraint on the propagation direction of the bulk waves engendered.

[0005] A first solution of the state of the prior art illustrated in FIG.
1 consists of studying a study area situated in said substrate by means
of the acoustic beam emitted by the emission source situated on the first
surface of said substrate. A receiver is then situated on a second
surface of the same substrate, generally in lamellar form, the first and
second surfaces being substantially opposite each other and parallel.
Such a technical solution means that the first and the second surfaces
are occupied, one by the emission source, the other by the receiver. The
emission source, the receiver and the study area are aligned.

[0006] It is often useful, in addition to the measurement by the acoustic
beam, to carry out another type of measurement on the study area. This
other type of measurement, for example via an observation of the study
area, requires the first or the second surface, preferably the second
surface, to be free so as to be able to have a second observation device
such as an optical microscope above the study area, aimed at the second
surface.

[0007] A second solution according to the prior art illustrated in FIG. 2,
making it possible to leave the second surface free, then consists of
having on the second surface, on either side of the study area, combs or
networks of lines with, for example, triangular or rectangular profiles
and at intervals equal to the wavelength of a created surface wave. On
reaching the second surface of the substrate, the acoustic wave emitted
by the emission source is transformed into an acoustic surface wave.
Thus, a surface wave is propagated over the second surface of the
substrate, crossing the study area. However, such a solution is reserved
for classes of materials that are both expensive and incompatible with
certain applications such as biology, in particular when the substrate
comprises lithium niobate (LiNbO3) or zinc oxide (ZnO). This
solution is moreover selective in frequency, and does not make it
possible to have a large interaction volume, since it is limited to a
wavelength in the thickness below the surface of the substrate.

[0008] The purpose of the present invention is to propose a device or a
method for studying a study area by means of an acoustic wave, that does
not have the drawbacks of the solutions of the state of the art set out
above.

DISCLOSURE OF THE INVENTION

[0009] This objective is achieved with a device for studying a study area
by means of an acoustic wave, comprising a substrate provided with
distinct first and second surfaces, characterized in that it comprises:

[0010] an emission source arranged to emit an acoustic beam originating
from the first surface in the direction of the second surface,

[0011] first reflection means arranged to reflect said beam originating
from the first surface, so that, after reflection, the beam has a
propagation direction substantially parallel to the second surface and
crosses the study area,

[0012] second reflection means arranged to reflect the acoustic beam in
the direction of the first surface after the beam has crossed the study
area, and

[0013] a receiver arranged to receive said acoustic beam reflected by the
second reflection means.

[0014] The invention makes it possible to have an acoustic beam with a
propagation direction substantially parallel to the second surface of the
substrate, and therefore to free the second surface for example to make
it possible to easily group together several sensors, and therefore
several functions, on the same circuit. In fact, the second surface is
left free, which makes it possible to arrange thereon a second device,
for example an observation and/or measurement device, for example an
optical microscope making it possible to view the study area.

[0015] Moreover, the first or second surface can be engraved in order to
produce elements thereon for the focussing (such as a lens), divergence
(such as a lens) or reflection (such as a mirror) of the acoustic beam,
these elements having an invariant form along an invariance axis
perpendicular to the first or second surface, which considerably
facilitates their production by standard lithography methods.

[0016] In a preferred embodiment, the emission source is situated on the
first surface of said substrate. By contrast, in another embodiment,
there may be an intermediate layer between the emission source and the
first surface of the substrate.

[0017] Similarly, in a preferred embodiment, the receiver is preferably
situated on the first surface of said substrate. By contrast, in another
embodiment, there may be an intermediate layer between the receiver and
the first surface of the substrate.

[0018] After reflection by the second reflection means, the acoustic beam
the propagation direction of which is parallel to the second surface may
be closer to the first surface than to the second surface, or may be
closer to the second surface than to the first surface.

[0019] Preferably, the first and second surfaces are opposite each other,
the substrate being for example parallelepipedic in form or having for
example the form of a disc.

[0020] In a preferred embodiment, the first and second surfaces are
substantially parallel. Preferably, the substrate is presented in the
form of a thin strip the two opposite faces of which for example having
the larger areas constitute said first and second surfaces.

[0021] The first and/or second reflection means of the device according to
the invention preferably comprise a mirror formed by an interface of
materials, preferably by an interface with the substrate, preferably an
air/substrate interface. Thus, a simple difference in index between two
materials at the level of an inclined mirror creates a deflection of the
acoustic beam emitted by the emission source towards the first reflection
means. This difference in index is for example the difference between the
index of the substrate and the index of the air, if the substrate is
immersed in air, but can also for example be the difference between the
index of the substrate and the index of a liquid in which said substrate
is immersed. The device according to the invention can also comprise at
least one additional layer of material or a set of several additional
layers of different superimposed materials, this layer or these layers
being arranged on the substrate, so that the first and/or the second
reflection means comprise a mirror formed by an interface of materials
between the substrate and this additional layer or this set of several
additional layers. Each additional layer is preferably selected from: a
layer of metal such as preferably copper, zinc, tin, titanium, or a layer
of dielectric material, preferably comprising silicon, preferably a
silicon oxide such as silicon monoxide SiO or silica SiO2. The
thickness of each additional layer is preferably comprised between 0.1
micrometre and 10 micrometres, more preferentially between 0.5
micrometres and 5 micrometres.

[0022] The substrate is for example silicon-based. It can be pure silicon,
or an alloy or mixture comprising in particular silicon. The substrate
can also non-limitatively comprise metal, glass, or a solidified polymer
such as polydimethylsiloxane (PDMS).

[0023] According to a preferred embodiment, the first and/or second
reflection means of the device according to the invention comprise a
recess formed on the second surface of the substrate. Each recess is
preferably engraved in the second surface of the substrate, and
preferably forms an interface with respect to the second surface, which
interface acts as an inclined mirror. Thus, the deflection of the
acoustic beam is created not only by a difference in index, but also by
the surface state of the substrate, for example its local inclination
with respect to the acoustic beam.

[0024] The first and/or second reflection means preferably comprise a
mirror inclined at 45° with respect to the second surface. By
mirror is meant, for example, an interface between two materials with
different indices. This interface is preferably inclined at 45°,
so as to deflect the acoustic beam typically by an angle of 90°,
or more generally by an angle equal to twice the acute angle between the
acoustic beam and the normal to the mirror. It is important to note this
is a reflection and not a transformation of an incident wave into a
surface wave. In this sense, the device according to the invention
differs from the second solution according to the prior art, in which the
wave being propagated in a direction substantially parallel to the second
surface is only a surface wave.

[0025] Preferably, the emission source and/or the receiver comprise
piezoelectric transducers. Piezoelectricity is the property possessed by
certain bodies, of becoming electrically polarized under the action of a
mechanical stress and reciprocally becoming deformed when an electric
field is applied to them. Thus, the acoustic wave being linked to the
notion of mechanical stress or vibration, a piezoelectric transducer
serves as an acoustic wave emission source, converting a voltage into a
mechanical stress, and as a receiver converting a mechanical stress into
a voltage.

[0026] According to a preferred embodiment, the study area comprises part
of a microfluidic channel preferably situated on the second surface. It
is thus possible to non-destructively and dynamically characterize the
mechanical properties of a biological cell passing through this
microfluidic channel and reached by the acoustic beam.

[0027] According to a preferred embodiment, the device comprises at least
one lens of a given form or curvature for example cylindrical or
parabolic, each lens being arranged in order to be passed through by the
acoustic beam. Preferably, the form or curvature of the at least one lens
does not vary along an axis substantially perpendicular to the second
surface, this axis hereafter being referred to as the invariance axis. In
one embodiment, the device according to the invention can comprise two
lenses, a first lens being situated on the path of the acoustic beam
between the first reflection means and the study area, a second lens
being situated on the path of the acoustic beam between the study area
and the second reflection means. In a variant, the device according to
the invention can comprise only one of these two lenses. In another
embodiment, the device according to the invention can comprise a concave
lens, the study area being situated in the middle of the concave lens.
Preferably, the at least one lens has the form of a portion of a cylinder
the axis of revolution of which is substantially perpendicular to the
second surface. Such a geometry has advantages in terms of ease of
manufacture, in particular by lithography methods. In fact, in this case,
if the device according to the invention is compared with the first
solution according to the prior art, the direction of the invariance axis
of the lens in the device according to the invention has the advantage of
being adapted to the plane of maximum resolution of the engraving
devices, namely the planes parallel to the surface of the substrate.
Here, these are the planes parallel to the second surface of the
substrate. According to the invention, it can be seen that the radius of
curvature of the at least one lens can then be engraved with precision,
whereas according to the prior art, a cylindrical lens would have an axis
parallel to the second surface of the substrate and the radius of
curvature of this cylindrical lens would then be engraved step by step,
gradually varying the engraving heights in the substrate. The engraving
is then carried out with less precision.

[0028] The arrangement of the second surface according to the invention
also makes it possible, for the same reason, to simply and precisely
engrave pillar or membrane type elements in the microfluidic channel so
as for example to obscure part of the incident or transmitted acoustic
beam, or to excite one of the vibration modes of these elements by
radiation pressure.

[0029] The at least one lens is preferably formed directly in the
substrate, so as to maintain continuity of material from the emission
source up to the study area in particular in order to limit energy
losses. A lens the form or curvature of which does not vary along an axis
substantially perpendicular to the second surface is advantageous as it
makes it possible both to concentrate the energy on a reduced surface
area which increases the resolution on the measurements in the study
area, and to scan the study area using an acoustic beam focussed on a
segment, which increases the probability of detecting for example a
molecule or a cell passing through the microfluidic channel compared with
the case of focussing on a point. It is nevertheless possible to envisage
using a spherical lens, which then focuses the acoustic beam
substantially on a point, but the engraving of which would be more
complicated.

[0030] If the study area is part of a microfluidic channel, the at least
one lens is preferably formed in a wall of the microfluidic channel.
Thus, a particularly compact device is produced since a recess is simply
made in the form of a portion of a cylinder, in at least one wall of the
microfluidic channel situated facing the study area.

[0031] According to a certain embodiment, the first reflection means are
distinct from the second reflection means, and the emission source is
preferably distinct from the receiver. In this embodiment, the acoustic
beam is emitted by the emission source, reflected by the first reflection
means, crosses the study area in the substrate, and is then reflected by
the second reflection means towards the receiver. This is referred to as
working in transmission, as the study area is studied as a function of a
beam that is transmitted inside this area.

[0032] According to another embodiment, the first and second reflection
means are merged. This is referred to as working in reflection. According
to this embodiment, the emission source and the receiver are preferably
also merged. Thus, the emission source emits the acoustic beam in the
direction of the first reflection means, then the acoustic beam is
reflected by the first reflection means, then it penetrates inside the
study area. There, either the device also comprises third reflection
means arranged to reflect the beam towards the second reflection means
(merged with the first reflection means) after the beam has passed
through at least part of the study area, or the study area is arranged to
reflect the beam towards the second reflection means. In this second
case, the acoustic beam is for example reflected on a cell or molecule or
other element passing through the microfluidic channel. It should be
noted that it is moreover possible to work in reflection without the
first and second reflection means being merged. A simple angular offset
between the optical axes of the first reflection means, third reflection
means and second reflection means for example makes it possible for the
beams incident and reflected on the study area not to be aligned, and
therefore for the first and second reflection means not to be merged. In
this case, the emission source and the receiver are not necessarily
merged either.

[0033] In a preferred embodiment, the second reflection means and the
receiver are arranged in order to receive a beam diffracted in the study
area. The device can in this case comprise several assemblies each
comprising second reflection means and a receiver, a first assembly
making it possible to study the acoustic beam in transmission,
substantially in the alignment of the incident beam in the study area, at
least one other assembly making it possible to study the part of the
acoustic beam diffracted by the study area. The device can comprise
either only the second reflection means and the receiver substantially in
alignment with the incident beam in the study area, or only the second
reflection means and the receiver substantially aligned with the beam
diffracted by the study area, or several reflection means and several
receivers in order to study both the beam transmitted substantially
without angular offset with respect to the incident beam in the study
area and at least one order of diffraction in the study area. The study
both of the transmitted beam and of at least one diffracted beam makes it
possible to have different contrasts in the study area, and to retrieve
more information on the latter. Such a device in which the second
reflection means are for example inclined with respect to the axis of the
microfluidic channel also allows for the study of a beam diffracted in
the study area, and the study of other types of beams deflected in the
study area, for example the study of a beam diffused in the study area.

[0034] According to another embodiment, the device comprises several
emission sources and several receivers, each receiver being associated
with one of the emission sources and being arranged to receive an
acoustic beam: said beam being emitted from the first surface and in the
direction of the second surface by the emission source with which it is
associated, then reflected by the first reflection means so that, after
reflection, the beam has a propagation direction substantially parallel
to the second surface and crosses the study area, then reflected by the
second reflection means in the direction of the first surface after
having crossed the study area.

[0035] According to a first associated embodiment, the emission sources
are aligned so that different acoustic beams emitted by the different
sources cross the study area at different heights. Each emission source
allows for the study of a given height of the study area or of the
substrate. It is thus possible to analyze different heights for example
of the microfluidic channel, but also to produce an interference contrast
by leaving part of the transmitted beam below the microfluidic channel.
For reception, several receivers are arranged in line, each preferably
receiving a beam emitted by a corresponding emission source.

[0036] According to another embodiment, the projections of the emission
sources and of the receivers on a plane parallel to the second surface
and crossing the study area are distributed around the study area.
Preferably, the first reflection means are then constituted by at least
one mirror surrounding part of the study area, have for example the form
of at least one arc of a circle centred on the study area, and make it
possible to reflect the acoustic beams emitted by all the emission
sources. Similarly, the second reflection means are preferably
constituted by at least one mirror surrounding part of the study area,
having for example the form of at least one arc of a circle centred on
the study area, and making it possible to reflect the acoustic beams
towards all the receivers. Such an embodiment makes it possible to study
the transmitted beam and/or the reflected beam as a function of an angle
for example between the acoustic beam reflected by the first reflection
means and the direction of flow in the microfluidic channel.

[0037] Preferably, the first and/or second reflection means comprise at
least one curved mirror. Preferably, the form or curvature of the at
least one curved mirror does not vary along an axis substantially
perpendicular to the second surface. In fact, according to an
advantageous embodiment, several emission sources are arranged preferably
in line, so that the total acoustic beam reflected by the first
reflection means is enlarged along an axis substantially perpendicular to
the invariance axis of at least one lens and/or along an axis
substantially parallel to the invariance axis of the at least one lens.
Thus, it is possible to direct all the more energy towards the study
area. It is then advantageous to concentrate this energy, in particular
by means of the focussing action of the at least one lens. However, the
operator is relatively limited for concentrating this energy. In fact,
the at least one lens being preferably formed in a side wall of a
microfluidic channel, the radius of curvature of said at least one lens
is fixed by the width of the microfluidic channel. This leads to a
determined pupil diameter D. By preferably using the first and/or second
reflection means comprising a curved mirror, it is possible to combine
the action of at least one curved mirror with the action of a lens. The
at least one curved mirror makes it possible to reduce the diameter of
the acoustic beam incident on a lens in order to adapt this diameter to
the pupil diameter D. The at least one curved mirror is for example a
cylindrical mirror, a parabolic mirror, or of a form optimized in order
to minimize geometric aberrations. Besides the energy concentration, an
additional advantage of this at least one curved mirror relates more
particularly to the embodiment in which different acoustic beams cross
the study area at different heights. The production of a network of N
emission sources provides access to selective acoustic information as a
function of the height z in the study area. This leads to an elementary
height for each of the emission sources equal to P/N, where P represents
the maximum height of the study area. The surface area of the emission
sources is then preferably fixed at D*P/N, which surface area can lead to
electric impedances completely different from and generally greater than
the characteristic impedance (often 50Ω). It is therefore
particularly advantageous to be able to further concentrate energy at
each altitude z in the study area. Whereas a lens the form or curvature
of which does not vary along an axis substantially perpendicular to the
second surface only concentrates the energy mainly on an axis parallel to
its invariance axis, the at least one curved mirror makes it possible to
also concentrate the energy on an axis perpendicular to its invariance
axis. Preferably, two curved mirrors are used, arranged afocally so as to
obtain a magnification greater than one between the acoustic beam
originating from the emission source and the acoustic beam incident on
the lens. The possibility of producing the at least one curved mirror is
a direct consequence of the propagation direction of the acoustic beam,
after reflection on the first reflection means, substantially parallel to
the second surface. Thus, it is possible to concentrate more energy on a
smaller surface area, inside the study area.

[0038] According to another aspect of the invention, a method according to
the invention is proposed for studying a study area by means of an
acoustic wave, implemented by a device according to the invention, and
characterized in that it comprises [0039] the emission of an acoustic
beam originating from a first surface of a substrate in the direction of
a second surface of this substrate, the first and second surfaces being
distinct, [0040] a first reflection by first reflection means, said first
reflection reflecting said beam originating from the first surface, so
that the beam has, after reflection, a propagation direction
substantially parallel to the second surface and crosses the study area,
[0041] a second reflection by second reflection means, said second
reflection reflecting the acoustic beam in the direction of the first
surface after the beam has crossed the study area, and [0042] the
reception, by a receiver, of the acoustic beam reflected by the second
reflection means.

[0043] The method according to the invention can also comprise the passing
of the acoustic beam through at least one lens, each lens being arranged
to be passed through by the acoustic beam between the first reflection
means and the study area and/or between the study area and the second
reflection means.

[0044] The method according to the invention can also comprise the
reception, by the second reflection means and by the receiver, of an
acoustic beam diffracted in the study area.

[0045] The method according to the invention can comprise several
emissions of acoustic beams by several sources and several receptions of
acoustic beams by several receivers, each receiver being associated with
one of the emission sources and receiving an acoustic beam emitted by the
emission source with which it is associated, then reflected by the first
reflection means, then reflected by the second reflection means after
having crossed the study area. Preferably, the different acoustic beams
emitted by the different emission sources cross the study area at
different heights.

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0046] Other advantages and features of the invention will become apparent
on reading the detailed description of implementations and embodiments
that are in no way limitative, and the following attached drawings:

[0047] FIG. 1 illustrates a perspective view of a first embodiment of the
device according to the prior art,

[0048] FIG. 2 illustrates a perspective view of a second embodiment of the
device according to the prior art,

[0049] FIG. 3A illustrates a perspective view of a first embodiment of the
device according to the invention,

[0050] FIG. 3B illustrates a cross-sectional profile view along the axis
MN of the first embodiment of the device according to the invention;

[0051] FIG. 4 illustrates a top view of a second embodiment of the device
according to the invention,

[0052] FIG. 5 illustrates a perspective view of a third embodiment of the
device according to the invention,

[0053] FIG. 6 illustrates a top view of a fourth embodiment of the device
according to the invention,

[0054] FIG. 7 illustrates a cross-sectional profile view along the plane
(Oxy) of a fifth embodiment according to the invention;

[0055] FIG. 8 illustrates a top view of first reflection means comprising
a curved mirror.

[0056] FIG. 1 illustrates a first embodiment according to the prior art in
which an emission source 1 emits an acoustic beam represented by the
arrow 2 through a substrate 5 from the first surface 7 of this substrate
towards the second surface 6 of this substrate. The acoustic beam then
crosses a study area 3 with a dotted outline. FIG. 1 also represents a
cylindrical lens 4 formed in the substrate 5. The planes of maximum
resolution of the engraving devices making it possible in particular to
form the cylindrical lens 4 are planes parallel to the second surface 6
of the substrate 5. It can therefore be seen in FIG. 1 that in the first
embodiment according to the prior art, the orientation of the cylindrical
lens 4 is not optimal with respect to this plane and that the latter must
be formed by successive engravings in planes parallel to the second
surface 6. This first embodiment according to the prior art has the major
drawback of not leaving the second surface 6 free since a receiver 10
would have to be placed above the second surface 6, which prevents
measurements and/or observations of the study area 3 other than those
carried out by means of the acoustic beam. In this case, the substrate 5
is for example lithium niobate (LiNbO3) or zinc oxide (ZnO).

[0057] FIG. 2 illustrates a second embodiment according to the prior art,
in which the emission source emits an acoustic beam represented by the
arrow 2 from the first surface 7 in the direction of the second surface
6. Networks of lines 8 with triangular profiles and at intervals equal to
the wavelength of the surface wave are arranged on both sides of the
study area 3. After the arrival of the acoustic beam on the network of
lines 8 situated on the side of the emission source 1, a surface wave 9
is propagated over the second surface 6 of the substrate 5, crossing the
study area 3. Such a solution is non-biocompatible, and also has a very
poor energy yield, since only surface waves are used in the study area,
instead of the main acoustic beam. Moreover, in this embodiment the
height of the interaction volume is limited to a wavelength under the
surface 6 of the substrate 5.

[0058] FIGS. 3 to 8 show different embodiments of the device according to
the invention implementing a method according to the invention. Each of
these embodiments comprises a substrate 5 provided with a first surface 7
and second surface 6, and an emission source 1 for emitting an acoustic
beam from the first surface 7 in the direction of the second surface 6.
The substrate is in the form of a thin strip in which the first surface 7
is parallel to the second surface 6. The beam emitted in the direction of
the second surface 6, before reaching the first reflection means 11, has
a propagation direction perpendicular to the first surface 7 and to the
second surface 6. The acoustic beam is represented by the arrows 2. The
first reflection means 11 are arranged in order to reflect the acoustic
beam originating from the emission source 1 so that the beam has, after
reflection, a propagation direction substantially parallel to the second
surface 6 and crosses the study area 3. Second reflection means 12 are
arranged in order to reflect the acoustic beam in the direction of the
first surface 7 after the beam has crossed the study area 3. The beam
reflected by the second reflection means 12 has a propagation direction
perpendicular to the second surface 6 and to the first surface 7. A
receiver 10 is arranged to receive the beam reflected by the second
reflection means 12. The substrate is made of silicon. The emission
source 1 is situated on the first surface 7. The receiver 10 is situated
on the first surface 7.

[0059] It should be noted that according to the invention, the reflection
of the acoustic beam does not change the nature of this beam but changes
its direction. In particular, the beam emitted is a bulk wave, and
remains a bulk wave after reflection by the first or second reflection
means 11, 12.

[0060] The emission source 1 is situated on the first surface 7. The
receiver 10 is itself also situated on the first surface 7.

[0061] The emission source 1 and the receiver 10 comprise piezoelectric
transducers. The surface area of a piezoelectric transducer is for
example ten thousand square micrometres. A piezoelectric transducer is
often presented in the form of a bar, for example with a main surface
substantially parallel to the first surface 7 with dimensions of one
hundred micrometres times one hundred micrometres, and with a height of
three micrometres. The study area 3 comprises part of a microfluidic
channel 13 engraved along the second surface 6 and through which pass in
particular biological cells 14, and with a height of one hundred
micrometres. The width l of the microfluidic channel 13 is for example
one hundred micrometres, not including the thickness of the lenses 4.

[0062] Each of the reflection means 11 or 12 shown in the figures
comprises a simple air/substrate interface engraved in the second surface
6 of the substrate 5.

[0063] Each interface is formed by a recess. The recess is in this case
substantially in the form of a prism, the base of which is substantially
parallel to the second surface 6 and comprising two faces inclined
substantially at 45° with respect to the second surface 6. One of
the faces forms a mirror inclined at 45° with respect to the
second surface 6. The prism has for example a length L that measures one
hundred micrometres and a height h that measures one hundred micrometres.
It can be seen in particular in FIG. 7 that the edge formed by the two
faces of the prism inclined at 45° with respect to the second
surface 6 do not necessarily meet directly but can be linked by an
additional face 14 for example substantially parallel to the second
surface 6.

[0064] For a frequency of the Gigahertz order in a silicon substrate, the
wavelengths used by the acoustic beam are of the micrometre order, for
example eight to nine micrometres. At these orders of magnitude, the
surface of the reflection means 11 and 12 does not require a very high
polishing, in contrast to the needs when working for example with light
beams in the visible range.

[0065] The receiver 10 is linked to analysis means that exploit the
electric signal generated by the receiver in response to the acoustic
beam received, and calculate different pieces of information from this
signal. For example, these analysis means can be arranged in order to
process the amplitude and the phase of the acoustic beam received by the
receiver, in order to quantify a speed of propagation of the acoustic
beam in the study area and/or an absorption of the acoustic beam in the
study area, then determine a coefficient of transmission or acoustic
reflection of the objects (biological cells) crossing the study area or
elastic properties of these objects. The processing of the acoustic beam
received typically comprises frequency amplification and filtering. The
analysis means typically comprise a computer equipped with an acquisition
adapter for the signal generated by the receiver, or a dedicated analogue
or digital circuit. For these different stages of processing,
quantification and determination, it is possible for example to implement
methods or algorithms as described in the following references: [0066] C.
F. Quate, A. Atalar and H. K. Wickramasinghe, Acoustic microscopy with
mechanical scanning--A review, Proc. IEEE, vol. 67, No. 8, pp 1092-1113,
1979 [0067] B. Hadimioglu and C. F. Quate, Water acoustic microscopy at
suboptical wavelengths, Appl Phys Lett, vol. 43, pp 1006-1007, 1983.

[0068] FIGS. 3A and 3B show a first embodiment, in which work is carried
out in transmission. In this case, the first and second reflection means
11 and 12 are distinct, similarly the emission source 1 and the receiver
10 are distinct. It can also be seen in FIG. 3A that this embodiment
comprises two second reflection means 12: one aligned with the emergent
beam substantially without angular offset from the first reflection means
11 for studying the study area 3 in transmission, and the other aligned
with the diffracted beam represented by the arrow 15 for studying the
study area 3 in diffraction. There are therefore in the device according
to FIGS. 3A and 3B two receivers 10, one for receiving the diffracted
beam, the other for receiving the beam having crossed the study area 3
substantially without angular offset. It is also possible to imagine a
device in which only the beam diffracted in the study area 3 would be
studied, or by contrast only the beam transmitted substantially without
angular offset. It can also be seen in FIG. 3A that two cylindrical
lenses 4 are formed in the substrate 5 on the path of the acoustic beam
to be passed through by the acoustic beam, a first lens between the first
reflection means 11 and the study area 3, a second lens between the study
area 3 and the second reflection means 12. The radius of curvature of the
cylindrical lenses 4 is for example one hundred micrometres, and their
height is for example one hundred micrometres. The two cylindrical lenses
4 are formed directly by a deformation of part of the side walls of the
microfluidic channel 13. Such a device is therefore particularly compact:
it has few interfaces and therefore low energy losses. Each cylindrical
lens 4 has the form of a portion of a cylinder the axis of revolution of
which is perpendicular to the first surface 7 and to the second surface
6, this axis of revolution being an invariance axis along which the form
and the curvature of this lens 4 do not vary.

[0069] FIG. 4 is a top view of a second embodiment of the device according
to the invention that is described only for its differences with respect
to the first embodiment in FIGS. 3A and 3B, and in which the study area 3
is studied only by means of the beam transmitted substantially without
angular offset. In FIG. 4, C1 represents the centre of curvature of
the first cylindrical lens 4, and C2 represents the centre of
curvature of the second cylindrical lens 4. F represents the common focal
point of the two cylindrical lenses 4. However, the focal points of the
two cylindrical lenses 4 are not necessarily merged, although a situation
in which the focal points are merged is optimum. The embodiment according
to FIG. 4 is afocal: the acoustic beam emitted by the emission source 1
is a parallel beam, this beam is then focussed inside the study area 3 by
the first cylindrical lens 4, then it is once again collimated by the
second cylindrical lens 4 so that a parallel beam reaches the receiver
10. This configuration makes it possible to focus the energy along a bar
substantially parallel to the axis of the two cylindrical lenses 4,
situated inside the study area 3, for example, but not necessarily, in
the middle of the study area.

[0070] FIG. 5 shows a third embodiment of a device according to the
invention that is described only for its differences with respect to the
first embodiment in FIGS. 3A, 3B, and in which the study area 3 is
studied in reflection. In this case, the first reflection means 11 are
merged with the second reflection means 12. The acoustic beam emitted by
the emission source 1 then reflected by the first reflection means is
reflected in the direction of the second reflection means by the study
area 3, for example by a cell 14 situated in this area 3. In a variant
which is not shown, the acoustic beam emitted by the emission source 1
then reflected by the first reflection means is reflected in the
direction of the second reflection means by third reflection means
situated for example on the side of the study area 3 opposite the first
reflection means 11, and aligned so as to send an acoustic beam towards
the second reflection means 12. Moreover, the receiver 10 and the
emission source 1 are merged.

[0071] FIGS. 6 and 7 illustrate embodiments comprising several emission
sources 1 identical to that previously described, and several receivers
10 identical to that previously described. Each receiver 10 is associated
with one of the emission sources 1 and is arranged to receive an acoustic
beam:

[0072] emitted from the first surface 7 and in the direction of the second
surface 6 by the emission source 1 with which this receiver is
associated, then

[0073] reflected by the first reflection means 11 so that the beam after
reflection has a propagation direction substantially parallel to the
second surface 6 and crosses the study area 3, then

[0074] reflected by the second reflection means 12 in the direction of the
first surface 7 after having crossed the study area 3.

[0075] FIG. 6 shows a fourth embodiment of a device according to the
invention that is described only for its differences with respect to the
first embodiment in FIGS. 3A, 3B, and making it possible to study the
study area 3 as a function of the angle of incidence of the acoustic beam
in the study area 3. The projections of the several emission sources 1
and receivers 10 on a plane parallel to the second surface 6 and crossing
the study area 3 are distributed around the study area 3. In FIG. 6, an
area 16 marked by dotted lines defines the area in which emission sources
1 and receivers 10 can be situated. The emission sources 1 and the
receivers 10 are coupled two by two for example for a study in
transmission, so that a receiver 10 is arranged to receive an acoustic
beam emitted by a corresponding emission source 1 and reflected
successively by the first and second reflection means 11, 12 as described
previously. In this embodiment, the first reflection means 11 comprise a
single mirror for the different emission sources 1, and the second
reflection means 12 comprise a single mirror for the different receivers
10. Of course, the microfluidic channel 13 is cut into the substrate 5,
which is why if the emission sources 1 are distributed on both sides of
the microfluidic channel 13, the first reflection means 11 are themselves
also distributed on both sides of said channel 13, and the same applies
to the second reflection means 12. It is possible that some of the
emission sources 1 are situated on one side of the channel 13, while
other emission sources 1 are situated on the other side of the channel
13. The first and second reflection means 11 and 12 are interchangeable,
reflection means being arranged over the whole of the free edge of the
microfluidic channel as shown in FIG. 6.

[0076] In a variant of this fourth embodiment, the first and second
reflection means 11, 12 are merged, and each receiver 10 is merged with
the emission source 1 with which it is associated, so that this variant
operates in reflection according to the principle described with
reference to FIG. 5.

[0077] FIG. 7 shows a fifth embodiment of a device according to the
invention that is described only for its differences with respect to the
first embodiment in FIGS. 3A, 3B, and in which several emission sources 1
and several receivers 10 are aligned on the first surface 7 along a
straight line passing through a projection of the study area on the first
surface 7. Each source and each receiver has a rectangular form, the axis
parallel to the longest sides of these rectangles being parallel to the
axis of the prism formed by the first reflection means 11. Thus, the
emission sources 1 are aligned so that different acoustic beams emitted
by different emission sources 1 cross the study area 3 at different
heights of this study area, the height being defined along an axis
perpendicular to the first and second surfaces. It is therefore possible
to study the study area 3 at different heights. In FIG. 7, the acoustic
beams emitted by the different emission sources 1 cross the microfluidic
channel 13. It can be seen in FIG. 7 that the thickness of the beams
varies substantially from one emission source 1 to the other. In this
case this chiefly provides a way of differentiating easily and visually
between the two beams. However, the emission sources 1 are not all
necessarily identical, and do not necessarily emit beams of identical
widths. According to a variant not shown, certain of the emission sources
emit an acoustic beam which passes under the microfluidic channel 13. It
is thus possible to produce an interference contrast by analyzing both
the information provided by at least one acoustic beam that has passed
through the microfluidic channel 13 and at least one acoustic beam that
has passed under the microfluidic channel 13.

[0078] In a variant of this fifth embodiment, the first and second
reflection means 11, 12 are merged, and each receiver 10 is merged with
the emission source 1 with which it is associated, so that this variant
operates in reflection according to the principle described with
reference to FIG. 5.

[0079] FIG. 8 shows a detailed top view of a variant of any embodiment of
a device according to the invention that has just been described in which
the first reflection means 11 comprise at least one curved mirror 21, 22.
In the device according to FIG. 8, the acoustic beam represented by the
arrow 2 is reflected by a substantially plane mirror 20 inclined at
45° with respect to the second surface 6 of the substrate 5. After
reflection on this substantially plane mirror 20, the acoustic beam has a
propagation direction substantially parallel to the second surface 6 of
the substrate. Two curved mirrors 21 and 22 are arranged afocally. The
two curved mirrors 21 and 22 are for example cylindrical, parabolic, or
of a form optimized in order to minimize geometric aberrations. The form,
in particular the curvature, of each curved mirror 21, 22 does not vary
along an axis substantially perpendicular to the second surface. C3
is the centre of curvature or parabolic focal point of the mirror 21.
C4 is the centre of curvature or parabolic focal point of the mirror
22. F12 is the common focal point of the two curved mirrors 21 and
22. Spatial filters 23 are situated on the path of the acoustic beam,
substantially at the level of the common focal point F12 of the
curved mirrors 21, 22, in order to filter out certain geometric
aberrations of the acoustic beam. The acoustic beam incident on the
curved mirror 21 has a diameter D1 in a plane substantially parallel
to the second surface 6, and a propagation direction substantially
parallel to the second surface 6 of the substrate. The incident acoustic
beam emergent from the afocal system formed by the curved mirrors 21 and
22 has a diameter D2 in a plane substantially parallel to the second
surface 6, and a propagation direction substantially parallel to the
second surface 6 of the substrate, where D2 is less than D1.
There is therefore a magnification factor D1/D2 greater than
one on the acoustic beam. It is therefore possible to concentrate more
energy inside the study area 3 by means of first reflection means 11
comprising a curved mirror. Similarly, if the second reflection means 12
comprise at least one curved mirror 21, 22 according to the same
arrangement as that which has just been described for the first
reflection means 11, it is possible to simply increase the diameter of
the acoustic beam received by the receiver 10 in the same way.

[0080] In a variant, the mirror 20 can be a curved mirror and can replace
or supplement the mirrors 21, 22.

[0081] As described previously, the beam emitted in the embodiments that
have just been described is a bulk wave, and remains a bulk wave after
reflection by the first or second reflection means 11, 12. In general,
there can be three bulk waves in the substrate or propagation media such
as silicon: [0082] a compression wave referred to as a longitudinal
wave, and [0083] two shear waves referred to as transverse waves.

[0084] An incidence at 45° of a longitudinal wave on an acoustic
mirror in contact with the air, which leads to a mechanically free
interface, makes it possible to reflect typically from 10% to 20% of the
incident amplitude of a longitudinal wave. The remainder is converted
into a shear wave.

[0085] Shear waves cannot be used directly for BioMEMS-type applications.
The need to use two mirrors in order to develop a measurement system in a
microfluidic channel makes it necessary, for BioMEMS, to find a solution
in order to improve the longitudinal wave reflection coefficient of the
mirrors and reflection means of the device according to the invention.

[0086] In other applications, by contrast, it is necessary to improve the
transverse wave reflection coefficient of the mirrors and reflection
means of the device according to the invention.

[0087] Thus, in an improvement of any one of the embodiments that has just
been described or any one of the variants that has just been described,
for at least one component of (preferably for each component of):
[0088] the first reflection means 11, [0089] the curved mirror 21, [0090]
the curved mirror 22, [0091] the second reflection means 22, and/or
[0092] the third reflection means, the reflecting surface of this
component arranged in order to reflect the acoustic wave comprises one
(or preferably several) additional layer(s), each additional layer
comprising a material judiciously chosen in order to considerably improve
the longitudinal or transverse wave reflection coefficient depending on
the use that is made of this wave. More precisely, the first and the
second reflection means 11, 12 comprise a mirror formed by an interface
of materials with the substrate, and the device according to the
invention comprises at least one additional layer of material or
preferably a superimposition of several additional layers of different
materials arranged on the substrate, so that the first and second
reflection means 11, 12 comprise a mirror formed by an interface of
materials between the substrate and this additional layer or this
superimposition of additional layers.

[0093] For each additional layer, there are several possible candidates
for materials depending on the performance sought. The following are
preferably used: [0094] an additional layer of metal such as preferably
copper, zinc, tin, titanium, or a combination of these metal layers,
and/or [0095] an additional layer of dielectric material, preferably
comprising silicon, preferably a silicon oxide such as silicon monoxide
SiO or silica SiO2, or a combination of these layers of dielectric
materials.

[0096] Each of the additional layers is either deposited by vacuum
evaporation, which is the preferred deposition method for an additional
metal layer, or by vacuum spraying, which is the preferred deposition
method for an additional layer of dielectric material.

[0097] The thickness of each additional layer is preferably comprised
between 0.1 micrometre and 10 micrometres, more preferentially between
0.5 micrometres and 5 micrometres.

[0098] In the variant in which the reflecting surface comprises several
additional layers, these additional layers are superimposed on top of
each other and are additional layers of different materials. By
superimposing several additional layers of different materials, the
bandwidth of the reflecting surface is enlarged with respect to the
frequency of the acoustic wave reflected by this surface.

[0099] In a specific embodiment example, where: [0100] the substrate 5
is made of pure silicon, and each of the reflection means 11 and 12
comprises a simple air/silicon substrate interface formed by a recess and
engraved in the second surface 6 of the substrate 5, [0101] each
interface reflects a longitudinal acoustic wave having a frequency within
a band from 0.1 GHz to 3 GHz, typically 1.6 GHz with an incidence of
45° to the interface, then each interface typically reflects 10%
to 20% of the incident amplitude of a longitudinal wave.

[0102] In a first variant, by depositing an additional layer of silica
SiO2 with a thickness of 4 μm, on each of the air/silicon
interfaces of the first and second reflection means, more precisely on
the outside of the substrate 5 i.e. on the air side, the reflection
coefficient of each interface becomes substantially equal to 80% for the
longitudinal wave within a band from 1.5 GHz to 1.7 GHz, with an
incidence of 45° to the interface.

[0103] In a second variant, by depositing an additional layer of silica
SiO2 with a thickness of 3 μm on each of the air/silicon
interfaces of the first and second reflection means, more precisely on
the outside of the substrate 5 i.e. on the air side, then an additional
layer of titanium with a thickness of 0.5 μm on each of the additional
layers of silica, then an additional layer of copper with a thickness of
3 μm on each of the additional layers of titanium, the reflection
coefficient of each interface becomes greater than 80% for the
longitudinal wave within a band from 1.56 GHz to 2.05 GHz, and with an
incidence of 45° to the interface. By increasing the additional
layers of different materials, the bandwidth of each of the reflection
means is thus improved.

[0104] Of course, the invention is not limited to the examples that have
just been described and numerous adjustments can be made to these
examples without exceeding the scope of the invention. It is moreover
possible to combine as desired the different embodiments previously
described.